CHEM 343 - Resonance Structures

Rules for making good and useful resonance structures + how to determine the major resonance structure for a molecule.

rules for a good resonance structure

• net charge is the same

• chemical formula is the same

• sigma bonds cannot change

• hybridization does not change

rules for a useful resonance structure

• no more than one charged carbon

• atoms to the right of carbon (N, O, F, Cl, Br) must have a complete octet

• carbon and atoms to the left of carbon can have 6 electrons (C, B, Al)

• 1st row of elements BCNOF cannot have more than 8 electrons

3 valid moves when crafting resonance structures

• lone pair → π bond

• π bond → lone pair

• π bond → π bond

(-) carbon on a structure

“push type” - e- flow away from (-) charge

(+) carbon on a structure

“pull type” - e- flow to the (+) charge

if you don’t see a charged carbon…

look for N, O, or S

resonance structures involving oxygen

“pull type” - if the π bond is TO the oxygen, e- flow TO the oxygen

“push type” - if the π bond iz 1 away from the oxygen, e- flow away from the oxygen

3 rules to determine which structure is major

1. octet rule

2. minimize number of formal charges

3. does charge make sense?